The present invention relates to an optical disk apparatus for recording information on or reproducing information from an information recording track of an optical disk by irradiating the optical disk with a light beam. More particularly, the present invention relates to focus error detection of a light beam in this optical disk apparatus. Also, the present invention relates to an optical disk processing method.
In recent years, various studies and developments have been made for the purpose of improving the recording density of an optical disk. In an optical disk apparatus, an attempt is made to reduce the spot size of an optical spot by increasing the optical NA (Numerical Aperture) or adopting a short-wavelength laser. When the optical NA is increased, problems of stricter specifications of the tilt of an objective lens and a short distance between the objective lens and optical disk surface are pointed out.
As is well known, the specifications of the tilt of the objective lens can be relaxed by decreasing the distance from the optical disk surface (light entrance surface) to the recording surface, i.e., the thickness of a substrate. A sample optical disk having a shorter distance than the existing DVD or the like (which has a substrate having a thickness of about 0.6 mm), e.g., adopting a substrate having a thickness of about 0.1 mm has been prepared.
On the other hand, a high NA also leads to a decrease in distance between the objective lens and information recording surface. The distance between the objective lens and optical disk in the existing DVD is 1 mm or more. However, when an NA higher than 0.8 is adopted, the objective and optical disk adjoin at a distance less than 0.2 mm. When the objective lens and optical disk adjoin via such a short distance, the most serious problem is a collision of the objective lens against the optical disk. Such collision takes place when focus control runs away upon a focus lead-in operation or upon mixing of any disturbance in light reflected by the optical disk due to the influence of scratches, fingerprints, and the like on the optical disk. Hence, a stable focus lead-in operation and a focus control system which has high resistance against the influence of scratches, fingerprints, and the like are demanded.
As a conventional method of avoiding such collision, a method of forming an arcuated pattern on the objective lens on the optical disk surface side is proposed (Jpn. Pat. Appln. KOKAI Publication No. 2000-20985). The objective lens with such a structure exploits the generation of a floating force with respect to the optical disk due to the air flow generated in the gap between the optical disk and objective lens upon rotation of the optical disk. The objective lens is passively aligned to a position where the floating force and the driving direction in the focus direction balance. However, in this method, since the floating force of the objective lens with respect to the optical disk changes depending on the rotational speed of the disk, the floating amount readily changes due to the influences of disk rotation variations. When the rotational speed of the disk is not constant, the objective lens becomes unstable in this control method.
In order to attain adequate focus control, servo control of a conventional optical disk apparatus, which uses a focus error signal of a focus system is preferable. For example, Jpn. Pat. Appln. KOKAI Publication No. 2000-011401 proposes a method of realizing stable focus lead-in operation by adding signals from focus error detection systems having two different focus detection ranges. According to this method, a stable focus lead-in operation is realized by adding a focus error signal obtained by a detection optical system that can assure a broad focus error detection range, and a focus error signal obtained by a detection system which has a narrow focus error detection range but can assure high detection sensitivity. At the same time, precise focus alignment is realized. However, when scratches, dust, fingerprints, and the like become attached to the disk surface, the amplitude of the focus error signal itself used as a servo signal becomes small, the signal is susceptible to disturbances, and so forth. Furthermore, when the NA is high, since the spot size on the disk surface becomes small, the signal is readily influenced by disturbances, and the focus servo consequently runs away. Such an unstable state similarly occurs even in a focus lead-in operation, thus disturbing a stable focus lead-in operation. That is, since two different focus errors are generated using return light from a single optical spot formed on the information recording surface of the optical disk, an unstable state of the optical spot formed by a high-NA optical system cannot be avoided. As a result, the conventional servo control is vulnerable to disturbances that depend on the surface state of the optical disk such as scratches, dust, fingerprints, and the like, and the focus servo readily runs away due to the influences of disturbances.
Once the servo runs away, the objective lens may collide against the disk to damage not only the optical disk but also the objective lens, and information recording/reproduction may be disabled. In this way, the factors necessary for realizing an optical disk apparatus compatible with high-density optical disks are to realize a focus error detection system with which a focus servo hardly runs away even under the influence of scratches, dust, fingerprints, and the like, and a focus error detection system which hardly collides against the disk, and to realize a stable focus lead-in operation at the same time.
As described above, when the conventional focus error detection circuit is applied to a high-NA optical system, the objective lens is highly likely to collide against the disk. Also, the focus servo itself inevitably runs away due to the influence of scratches, dust, and fingerprints attached to the disk. When the focus servo runs away, since the distance between the objective lens and disk is small, the objective lens collides against the optical disk.
Furthermore, in the case of a high-density optical disk having a two-layered information recording surface, a focus error detection range is inevitably narrowed down. For this reason, focus control performance deteriorates. When focus control fails, the objective lens inevitably collides against the optical disk. For this reason, it is indispensable to realize a stable focus lead-in operation.
Moreover, in an optical disk having a thin substrate, since individual optical disks have different substrate thicknesses, substrate thickness variation correction such as spherical aberration correction or the like must be performed. Hence, a control system which executes focus control while making spherical aberration correction that influences focus control must be realized.